45. They are being smuggled and also sold legally to Mexican authorities

The smuggled ones are semi auto and the legal ones can be select fire if requested by Mexico.

The market is for Mexico and the cartels for semi autos not around the world in combat zones.

A few years ago Mexico made a claim that over 100 illegal M16's were found and confiscated
by Mexican authorities , The weapons were found in a raid .

They being Mexico claimed they were smuggled in over the border.

The US government got involved and asked for the serial numbers of the weapons .
They were traced back to Colt manufacturing , the irony of the whole thing was these
weapons that were selective fire were sold by Colt to the Mexican government then resold by corrupt officials.

The AR15 semi auto is not high on the list for smuggled weapons if they are being bought specifically for combat zones
They would be expensive compared to a Russian , or Chinese AK47's
Also the parts needed if the AR15 breaks down is not readily available like an AK 47 variant in those parts of the world.

edit to add
I'm not saying you won't find any in third world combat zones but the numbers are very few